style: apply CSharpier formatting to all C# sources

Repo-wide sweep with the pinned CSharpier 1.3.0 tool. Whitespace and
line-wrapping only; OpenNest.Engine.Tests (109) and OpenNest.IO.Tests
pass after reformat, full solution builds 0 errors.

Added .csharpierignore so csproj/config XML keeps its existing layout
(CSharpier's XML wrapping churns attributes with zero benefit).

Formatting is now enforceable: dotnet csharpier check . passes.

OpenNest.Core/Nest.cs

@@ -1,7 +1,7 @@
-using OpenNest.Collections;
-using OpenNest.Geometry;
-using System;
+using System;
 using System.Collections.Generic;
+using OpenNest.Collections;
+using OpenNest.Geometry;
 
 namespace OpenNest
 {
@@ -11,9 +11,7 @@ namespace OpenNest
         public DrawingCollection Drawings;
 
         public Nest()
-            : this(string.Empty)
-        {
-        }
+            : this(string.Empty) { }
 
         public Nest(string name)
         {
@@ -128,7 +126,7 @@ namespace OpenNest
                     EdgeSpacing = EdgeSpacing,
                     PartSpacing = PartSpacing,
                     Quadrant = Quadrant,
-                    Quantity = 1
+                    Quantity = 1,
                 };
             }
         }